This invention relates to fuel cell generator systems, and in particular to a method and apparatus for dissipating electrical energy in fuel cell generator systems.
As described in U.S. Pat. No. 6,007,930, Adams et al., incorporated herein by reference, fuel cells generate direct current electricity from chemical fuel and oxygen. A fuel cell includes an anode that receives a fuel containing hydrogen, a cathode that receives a gas containing oxygen, such as air, an electrolytic layer between the anode and cathode that conducts positively charged ions, and a catalyst that facilitates the chemical reaction. As the voltage generated by a single fuel cell can be relatively small compared with the requirements of many electrically powered devices, such as an electrically powered vehicle, individual fuel cells are typically combined into one or more fuel cell stacks.
Fuel cell stacks can be used to power vehicles with electric motors. U.S. Pat. No. 5,780,981, Sonntag et al., incorporated herein by reference, discloses a process for controlling the electric power generated by a fuel cell of a vehicle as a function of an accelerator pedal position, by adjusting the rotational speed of an air compressor that supplies air to the fuel cell.
Vehicles that incorporate fuel cells may also employ a regenerative braking system which allows the electric motor to function as a generator during vehicle braking, thereby converting kinetic energy of the vehicle into electric energy. Such regenerated energy may be stored in a device for storing electric power, such as a battery, for discharge at a future time. U.S. Pat. No. 5,345,761, King et al., incorporated by reference herein, discloses such a regenerative braking system.
When a driver releases the accelerator pedal to slow a vehicle, it is desirable for the vehicle to decelerate at the same rate for each event under the same conditions for driver comfort and safety. A regenerative braking system can affect deceleration characteristics.
Some of the electric current produced by regenerative braking can be used to run vehicle electrical loads. In the case where less than a fully charged battery is available, the braking energy can be used to recharge the battery. However, in a vehicle powered by a fuel cell generator any regenerated electrical power that is not used to power electrical loads or to recharge a battery must be dissipated. This is because directing an electrical current into a fuel cell will damage or destroy the cell. Therefore, a difficulty may arise in vehicles that incorporate both fuel cell generator systems and regenerative braking systems since the electricity generated must not flow into the fuel cells of such systems.
In addition, fuel cell generator systems require the draw of one or more electrical loads at least equivalent to the minimum amount of electrical power that the fuel cell system can generate. Otherwise, an insufficient amount of fuel and oxygen will be supplied to the fuel cell generator system, causing the fuel cell system to cease operation and requiring it to be restarted when additional electrical power is required.
One solution to the problem of dissipating excess energy is to employ a resistor to dissipate electrical power. U.S. Pat. No. 6,025,083 Veyo et al., for example, discloses an apparatus and method for dissipating excess energy of a fuel cell through a resistor. However, such a method may require additional considerations such as providing a cooling system to control the heat energy produced by the resistor. Moreover the large size and expensive cost of such resistors further impede the efficiency of such a system.
Accordingly, it is desirable to provide a method and apparatus to dissipate electrical energy in a fuel cell generator system.